1. Field of the Invention
This invention relates to sensors used in differential thermal analyzers such as differential scanning calorimeters.
2. Background of the Invention
Differential thermal analyzers measure the difference in temperature between a sample material and a reference material, as the sample and reference materials are subjected to dynamic controlled changes of temperature. Measurement of the dynamic temperature difference as a function of temperature provides qualitative information concerning physical transformations which occur in the sample material. Differential scanning calorimeters are differential thermal analyzers which provide quantitative information about physical transformations occurring in the sample by measuring the flow of heat to or from the sample.
Differential scanning calorimeters fall into two broad classes of instruments: heat flux instruments and power-compensated instruments. Heat flux differential scanning calorimeters measure the dynamic temperature difference between a sample material and a reference material. Because the dynamic temperature difference is proportional to the heat flow to (or from) the sample, the heat flow to (or from) the sample is obtained from the dynamic temperature difference. Power compensated differential scanning calorimeters control the flow of heat to the sample material and reference material separately. The flow of heat is controlled so as to maintain the temperature of the sample material at the temperature of the reference material during physical transformations in the sample material. The heat flow to (or from) the sample material is calculated from the difference between power supplied to the sample material and the power supplied to the reference material.
Heat flux calorimeters comprise means for supporting the sample and reference materials, a temperature sensor to determine the temperature of the sample material, a differential temperature sensor to measure the difference between the temperature of the sample material and the temperature of the reference material, and a controlled-temperature enclosure. U.S. Pat. No. 4,095,453 to Woo describes a typical means for supporting a sample or reference material in a differential scanning calorimeter. The most typical type of support means is a circular disk but this is by no means the only configuration used. For example, some high sensitivity differential scanning calorimeters have receptacles mounted on short columns attached to the disk to hold the sample pans. The columns increase the heat flow resistances between the disk and the sample pans, thereby increasing the temperature difference between sample and reference and hence the sensitivity of the differential scanning calorimeter. However, this decreases the resolution in proportion to the increase in sensitivity. The enclosure may be supplied with a cooling device to more accurately control its temperature or for below-room temperature measurements.
Most instruments use thermoelectric effect differential temperature sensors, i.e., thermocouples, wherein a difference in the measurement temperature and a reference temperature generates an electromotive force proportional to the temperature difference between the measuring and thermoelectric reference temperatures.
There are two types of commonly-used differential temperature sensors based upon the thermoelectric effect. The differential thermocouple uses a single differential temperature-sensing element. The differential thermopile uses multiple, in-series differential thermocouples. U.S. Pat. No. 3,554,002 to J.C. Harden et al. describes a differential thermal analysis cell using a differential thermocouple. U.S. Pat. No. 5,033,866 to Kehl et al. describes a thermal analysis sensor using a differential thermopile.
The resolution and calorimetric sensitivity of an instrument are two of the important performance criteria for differential scanning calorimeters. These criteria serve to define the applications for which the instrument is best suited. Resolution is the instrument's ability to separate thermal events which occur at temperatures which are close to each other. It is determined by the dynamic thermal response of the instrument. Calorimetric sensitivity is the measure of the signal generated by an instrument in response to a thermal event having a certain heat flow. Dynamic thermal behavior is characteristic of the geometry of a particular instrument, and of the materials used in its construction.
The sensitivity of heat flux instruments is dependent upon the output of the differential temperature sensor, which is in turn dependent upon, in part, the temperature difference developed between the sample material and the reference material. This temperature difference is also dependent upon the instrument geometry and construction materials. However, there is an inverse relationship between instrument designs that improve resolution and instrument designs that improve sensitivity. For an instrument using a given type and configuration of differential temperature sensor, increasing sensitivity results in decreased resolution while increased resolution results in decreased sensitivity.
Thermopile instruments are capable of greater sensitivity, because the output signal of the sensor is increased in proportion to the number of thermocouples in the thermopile. However, geometric considerations eventually limit the performance of the instrument, because the number of differential thermocouples which fit into an instrument of a given size is limited.
Another very important measure of performance of differential scanning calorimeters is their signal-to-noise ratio. This ratio is a measure of the smallest heat flow detectable by the instrument. Noise in differential thermal analyzers is predominantly electromagnetic and amplifier noise. Sensors with low electrical impedance pick up less noise than sensors with high electrical impedance. Differential thermopile sensors have inherently high electrical impedance. Thermopile sensors are therefore highly susceptible to electromagnetic noise and thus have a decreased signal-to-noise ratio.
The heat flow is measured as a function of the sample temperature. The preferred method for measuring the temperature of the sample is to directly measure the sample temperature using a sample temperature sensor. An alternate but less desirable approach is to use the temperature of the enclosure as a measure of the sample temperature. U.S. Pat. No. 4,095,453 discloses an instrument that directly measures sample temperature. U.S. Pat. No. 5,033,866 discloses an instrument wherein the sample temperature is inferred from the enclosure temperature.
U.S. Pat. No. 4,350,446 describes a heat flux differential scanning calorimeter which can measure multiple samples simultaneously. Although this instrument improves productivity, in practice its calorimetric accuracy and precision is not equal to that of single-sample instruments. The user of such instruments must choose between accuracy and precision, and productivity, or must use separate single-sample and multiple sample instruments.
U.S. Pat. No. 3,263,484 describes power compensation differential scanning calorimetry. Power compensation calorimeters measure the difference in power supplied to the sample material and the reference material, and determine the heat flow from this measurement. Heat flux instruments calculate the heat flow from a differential temperature measurement. However, power compensation instruments are more complex than heat flux instruments.
Moreover, current power compensation instruments do not directly measure sample temperature.
Furthermore, there are no current instruments that can be used either as power compensation instruments or as heat flux instruments. A user who wants to practice both techniques would have to purchase instruments of each type.
In traditional differential scanning calorimetry, the sample material and the reference material are simultaneously subjected to the regulated temperature environment. However, this is not essential to the operation of the calorimeter. Differential scanning calorimetry may be performed sequentially, by subjecting the sample material and the reference material to consecutive measurements, storing the results, and subsequently calculating the heat flow to (and from) the sample. U.S. Pat. No. 4,848,921 describes this technique in power compensation calorimeters. In principle, heat flux calorimeters could also measure the heat flow to (and from) the sample material and the reference material sequentially.
Differential thermocouples and thermopiles, in addition to being used to measure temperature difference, may transfer energy between the measuring and reference points by employing the Peltier effect. The Peltier effect describes the behavior of a thermocouple circuit when direct electric current is applied to it. When an electric current is applied to the differential thermocouple, one of the two thermocouple junctions is heated and the other is cooled. This difference in temperature corresponds to a flow of heat from the colder to the hotter junction. Thus the Peltier effect refers to the use of a thermocouple as a heat pump. The magnitude of the Peltier effect is strongly dependent on the electrical impedance of the thermocouple: for a given type of thermocouple, thermocouples with low electrical impedance have greater heat pumping capacity than those with higher impedance. The current supplied to the differential thermocouple or thermopile is a measure of the heat pumped by the Peltier effect, as described in U.S. Pat. No. 4,451,690.